The present invention generally relates to electron beam tomography (EBT) systems, and more particularly to a method of adjusting electron beam deflection in order to stabilize relative x-ray intensities between detector rings in EBT systems.
Computerized tomography (CT) systems produce planar images along imaginary cuts, or slices, through a patient. CT systems typically include an x-ray source, which revolves about an imaginary axis through a subject. After passing through the subject, the x-rays impinge on an opposing array of detectors.
Typical CT patient scans are executed in either an axial mode or in a helical mode. In axial mode, the table that supports the patient stops, the scan is executed, and then the table moves to a new location. In helical mode, the patient table continuously moves throughout the course of the scan. Single slice scanners (scanners having one detector array) are common, and dual slice CT systems (systems having two detector arrays) are known.
Some CT scanners include a scanning electron beam x-ray source, such that an electron beam is magnetically deflected so as to rotate in a generally arcuate path, and in doing so, impinges upon an arc-shaped target. As the electron beam impinges on the target, a source of x-rays is generated. As the electron beam moves, so does the source of x-rays. The x-rays encounter a collimator that passes a portion and blocks a portion of the x-rays. The x-rays are shaped into a fan beam by the collimator and then intercepted by a ring-shaped detector array on an opposite side of the patient. U.S. Pat. No. 4,352,021 (“the '021 patent”), issued Sep. 28, 1982, discloses such an electron beam scanner.
With respect to dual detector arrays, the quality of images typically depends, at least in part, upon the position of the x-ray beam spot on the target. Typically, with dual detector arrays, it is desired to maintain uniform x-ray intensity on both detector arrays. Movement of the beam spot on the target affects the x-ray intensity on each detector. The beam spot may move due to several reasons including deformation of the target. For example, the deformation of the target with increased imaging sweeps causes the beam spot to move relative to the target. As the beam spot moves, or shifts, on the target, the resulting x-ray fan beam shifts in response thereto. As the x-ray fan beam shifts, each detector array that detects the x-ray fan beam receives more or less of the x-ray fan beam, depending on the nature of the shift. Consequently, the x-ray intensity on one detector array typically differs from the other detector array when the x-ray beam spot moves, or shifts, relative to the target.
Various phenomena may cause the beam spot to move relative to the target, including target deformation caused by thermal effects, eddy currents, and the like. As a scan progresses through multiple sweeps, the shifting effect of the beam spot relative to the target typically increases. Consequently, the position of the beam spot on the target at a particular point varies from sweep to sweep. As the beam spot moves, the x-ray fan beam is displaced with respect to the detector arrays and the resultant x-ray intensities on the detectors arrays varies, thereby producing images of varying quality, particularly at the end of long scans.
FIG. 1 illustrates an axial cross-sectional view of an electron beam tomography (“EBT”) system 100. The EBT system 100 includes a target ring 112 onto which an electron beam impinges at a beam spot 114, thereby producing an x-ray fan beam 116 that is detected by detector arrays 118. The electron beam may be rotated from one end 120 of the target ring 112 to the other end 122 of the target ring 112 through a semi-circular arc defined by the target ring 112. One “sweep” is typically defined by movement of the electron beam from one end 120 to the other end 122 of the target ring 112. With each passing sweep, however, the target ring 112 gradually deforms. The effect of deformation after numerous sweeps is shown by expanded ring 124, the shape of which is exaggerated for illustrative purposes. The effect of the deformation compounds with each passing sweep. Thus, after a large number of sweeps, the target ring 112 may be substantially deformed. As the target ring 112 deforms with each sweep, the resulting electron fan beam shifts, resulting in unequal x-ray intensities on the detectors.
Thus, a need exists for a method that ensures that the x-ray intensities on both detector arrays remain constant in order to provide consistent, high quality images.